1. Field of the Invention
This invention relates generally to a multistage method for making hollow plastic articles which includes molding preforms or other articles and blow-molding preforms into finished plastic articles and apparatus for this method, and particularly to a method and apparatus for making hollow plastic articles, such as molecularly oriented plastic bottles.
2. Description of the Prior Art
In recent years, substantial effort has been directed to the production of plastic bottles used as a replacement, or partial replacement, for glass bottles. The use of such bottles results from the fact that plastic, such as polyethylene terephthalate (PET), when molecularly stretched, is tougher but lighter than glass.
According to the prior art, molecularly oriented containers, such as plastic bottles, have been produced using either a two-stage process or a one-stage process. In the two-stage process, a preform or parison resembling a thick test tube is first injection molded in a parison mold in an operation entirely separate from the blow-molding step. The preform has a completely finished neck with all necessary threading and a neck ring to facilitate downstream handling. The preform, which is about seven inches long for a two-liter bottle, is cooled to room temperature, and stored for subsequent blowing into the finished bottle in a blow mold. At the time of blowing, the preform is fed into a reheat-stretch-blow-molding machine, where the preform is heated, mechanically stretched longitudinally (generally by means of an extensible core pin pushing against the bottom of the preform), and expanded or blown with compressed air. The longitudinal stretching and blowing biaxially orient the PET molecules, contributing to improved strength, barrier properties, and clarity.
The one-stage process incorporates the preform injection molding and stretch-blow-molding process steps in the same machine. The sequence is basically the same, except that there is less cooling and reheating of the preform. The preform is injection-molded in a parison mold, removed from the mold at maximum temperature, transferred directly through one or more temperature conditioning stations to achieve the proper blowing temperature, and then transferred at the blowing temperature to a blow mold where the preform is blown into the finished bottle.
Two-stage processes have also been developed in which the preforms are extruded, cut to a prescribed length, reheated in an oven, and fed into the stretch-blow-molding machine, in which in a single operation, the neck is formed, the preform is mechanically stretched and blown, and the bottom is pinched off. One-stage processes have also been developed in which a parison is extruded, blown into a preform in one mold, after which the preform is biaxially stretched and blown into a finished product in another mold, all of these operations taking place in a single machine.
Various types of single-stage blow-molding methods and apparatus for the formation of different types of plastic articles are disclosed in the following U.S. Pat. Nos.: 3,555,598 issued to Mehnert; 3,596,315 to Yoshikawa et al; 3,694,124 to Saumsiegle et al; 3,738,788 to Langecker; 3,887,316 to Hestehave; 3,940,223 to Farrell; 3,947,180 to Neumaier; 3,984,513 to Mulraney; 3,986,807 to Takegami et al; 3,963,404 to Polock et al; 4,140,464 to Spurr et al; and 4,140,468 to Duga.
Two-stage processes and apparatus for the formation of hollow plastic articles from preformed parisons are shown in many patents including U.S. Pat. Nos. 3,079,637 issued to Marzillier and 3,583,031 issued to Kader et al.
Both the one-step and the two-step processes have certain advantages and disadvantages. Among the advantages of a two-stage process is that efficient and effective use can be made of both the parison-forming and blowing stations which need not be integrated. The preform may be molded at a central location and shipped to several plants for blowing and filling. Preforms for a 64-ounce carbonated beverage bottle take up only 11 cubic inches of shipping space, less than 5 percent of the volume occupied by the finished bottle. Preforms may thus be produced at one location by an experienced custom molder and supplied to a beverage bottler in another location who may operate the blow-molding machine without prior knowledge of melt processing of plastic. However, substantial thermal energy is lost during the total operation, since the preform after formation is cooled down during storage and then reheated at the time of blowing. Moreover, the preforms from storage must be fed to the blowing station, duplicating handling. The one-stage process eliminates heat loss and duplicate handling. About 50 percent less energy per bottle is consumed during the reheating portion of the process. In addition, in the one-stage process, a continuous mechanical grip is maintained on the neck of each bottle from the preform molding stage to the stretch-blow-molding stages. This means that there is no need to release and regrip the bottle, eliminating a possible source of distortion and disfiguration due to contact between the preform and other bodies. In addition, the process maintains the preform temperature very high and achieves a more uniform temperature throughout the wall thickness of the preform, permitting low-pressure blowing with accurate dimensional control reducing processing costs and improving product quality. However, the advantages of the one-stage process are mitigated since, conventionally, the individual components of the system are constructed and arranged as an integral unit, and efficient use of the various system components is not realized, nor are the systems as rapid as desired in commercial manufacture. For each preform station, there is a corresponding blowing station. Since the time required for the preform formation stage is substantially longer than the blowing stage, inefficient use is made of the blowing station.
An additional disadvantage occurs in both one-stage and two-stage processes. In either process, the portion of the machine used to melt the plastic and inject it into the preform mold operates sequentially and is idle during a major portion of its cycle. Thus, one of the important portions of the apparatus is inefficiently used.
As the popularity of certain plastic articles, particularly PET plastic bottles used in the soft drink industry, has increased, the need for increased production of plastic articles has increased. There has been a need for a method for producing plastic articles which will yield production rates higher than those possible with the prior art. At the same time, it has been highly desirable to develop a system which is energy-efficient so that the amount of energy lost during the process is maintained at a minimum. Furthermore, it is highly desirable that all of the components of the apparatus are utilized to their capacity for the maximum amount of time to minimize the equipment investment.